1. Field of the Invention
The present invention relates to a method and apparatus for printing a full-color toned image based on the scheme of electrophotography, and particularly to an improved method and apparatus for printing a full-color toned image by forming mono-color images by means of mono-color imaging units provided separately for individual color components of full-color images, and overlapping the mono-color images.
2. Description of the Prior Art
Conventional full-color printing apparatus including full-color copy machines and full-color printers based on the scheme of electrophotography have used the technique of single-lined screen, which is generally known in the field of printing, for printing full-color toned images. The single-lined screen used for these machines has been of the type called "constant-angle single-lined screen" in which the screen angle is constant for all color components, e.g., yellow, magenta, cyanine (and black), of full-color images.
Many of these full-color printing apparatus are designed to attain the single-line structure by writing an image based on the raster scanning of a laser beam so that image information of adjacent rasters is continuous in the ancillary scanning direction. The machines have generally employed the "90.degree. single-lined screen", which is named as such because the screen angle is 90.degree. with respect to the main scanning direction.
However, the conventional full-color image printing apparatus of "tandem type", in which images of individual color components are formed by means of separate mono-color imaging units and these mono-color images are overlapped on a sheet of print paper, have suffered the disparity of imaging characteristics and positional accuracy among the mono-color imaging units. Therefore, in many cases, a produced full-color image has degraded accuracy of color registration (write position of each mono-color image) in the main scanning direction and the single-lined screens of all color components do not overlap precisely, resulting in the fluctuation of visual color.
In order to solve the above-mentioned technical problem, there is a method based on the scheme of screen rotation known in the field of printing, in which the screen angle is made different among the color components so that the single-lined screens of all color components overlap at random thereby to make even the produced color (refer to Japanese Patent Unexamined Publication No. 53-19201).
In this case, screen angles other than 90.degree. can be attained by shifting certain values specific to individual color components, at every raster scanning with respect to the usual constant-angle single-lined screen, and screen rotation of different screen angles for individual color components can be accomplished.
However, when this method is used to vary the screen angle greatly (e.g., a screen angle of 45.degree. by setting the shift value equal to a half line pitch of the screen), the number of lines increases by 1/sin.alpha. (where .alpha. is the screen angle, i.e., 45.degree.) relative to the original screen, i.e., a 1.41-fold increase in the number of lines in this example and therefore it necessitates a higher resolution of imaging. The number of lines of the single-lined screen is already set as high as possible within the ability of the apparatus in general with the intention of achieving the highest picture quality based on the highest resolution, and therefore the above-mentioned screen rotation demands much higher resolution beyond the ability of the apparatus and it can result in a degraded picture quality such as the degradation of tone quality.
Moreover, a great difference in the line pitch among color components results in a great difference in resolution among mono-color images, and the quality of full-color image produced by overlapping the mono-color images will be deteriorated, e.g., mono-color images of lower resolution will be emphasized.